AC commercial power is often used as a primary power source to power communication and data processing equipment which utilize solid-state integrated circuit technology. These circuits are very sensitive to variations of the power signal from its desired standard waveform. Commercial AC power waveforms are subject to many variations from the standard waveform due to the demands of other users on the power line and other extraneous factors.
Undesirable power signal variations causing problems include overvoltage and undervoltage conditions, signal outages, and signal transients such as voltage spikes. These power signal variations may alter the stored data or the switching signals and in extreme cases may damage the solid-state circuitry. Transient and momentary outages may cause undetected damage in data areas which are not immediately apparent and eventually cause costly shutdowns due to damaged circuitry, disrupted communications, or introduce errors in computations.
To avoid these aforementioned problems, uninterruptible power supplies are utilized to isolate variations in the AC power signal from the equipment being powered and to supply continuous power to an output regardless of the actual performance of the basic commercial AC power signal.
One uninterruptible power supply system providing signal variation and signal outage protection to solid-state equipment is disclosed by H. Fickenscher et al in U.S. Pat. No. 4,010,381, issued Mar. 1, 1977, and assigned to the same assignee as the instant application. The uninterruptible power supply disclosed therein utilizes a three-port power coupling medium to couple two periodic power sources to a single load to be continuously energized. This coupling medium, as shown in FIG. 1, is embodied as a double-shunted, ferroresonant transformer 10 with two input windings 11 and 12 isolated from each other and each loosely coupled to a single output winding 13. Separate and independent power sources are connected to each of the two input windings 11 and 12. These power sources are operated in a cooperative manner to supplement each other as needed to maintain continuous power at the output winding. The load 14 coupled to the output winding 13 ideally does not distinguish between which of the two power sources is actively supplying power, whether singly or in combination, since the ferroresonant transformer arrangement provides isolation between the power sources and the load.
Under the direction of a central controller 15, as shown in FIG. 3, several modes of operation of an uninterruptible power supply 20 are possible. These modes are defined by the power sources selected either singly or in combination to supply power to the load. The uninterruptible power supply disclosed by Fickenscher et al normally operates in one of three distinct modes: a normal mode where a first external power source, normally commercial AC connected to the first input winding 21, supplies all the output power to the load 24; an internal mode where normally a battery-energized inverter 16 connected to the second input winding 22 supplies the output power to the load; or, third, a share mode where both power sources connected to both input windings 21 and 22 equally contribute to the output power supplied to the load 24. The share mode may operate for a substantial duration of time during periods when the commercial AC power is substandard, or merely during a transitory interval while switching between the normal and internal modes.
The ferroresonant transformer power medium of Fickenscher et al may be theoretically considered to be an inductively coupled system with an equivalent circuit as shown in FIG. 2. In such an inductively coupled system power flow from either input 31 or 32 to the output 33 is a function of the phase displacement between the signal at the input and the signal at the output. Hence, power flow from any input to the output of the coupling medium is regulated by controlling the phase angle at that input in an appropriate manner.
In the Fickenscher et al uninterruptible power supply control of the power derived by the output from the two sources is dependent upon the relative periodic signal phases of the commercial AC signal, the inverter signal output, and the output power signal. These signal phases are controlled by a signal phase control 15, as shown in FIG. 3, to achieve the desired selected modes of operation. If the commercial AC signal applied to winding 21 and the inverter output signal applied to winding 22 are in phase, they both supply power to the output winding 23.
In the normal mode of operation, however, it is desirable that the commercial AC signal supply all the power, and the inverter 16 supply no power to the output load 24. To achieve this desired condition during the normal mode, the inverter output signal must be in phase with the output power signal. Inverter 16 is run continuously and, whether it delivers power to the output or not, is totally dependent upon the relative phase angle of the inverter output signal at winding 22 and the output signal at winding 23. This phase angle control in Fickenscher et al is achieved through a transformer sensing winding 25 which senses the phase of the output power signal. Control circuitry 15 generates appropriately phased switch drive signals for switching devices of inverter 16 whereby the inverter output signal is brought into phase with the output signal on winding 23 so that it contributes no power to the output signal during the normal mode of operation. This idling condition improves the efficiency, and yet permits the uninterruptible power supply to switch sources rapidly. Control of power output of the reserve power source in its idling condition is critical to the ultimate overall efficiency of the uninterruptible power supply. Such a control should have a fast response and a high degree of accuracy.
Another method of controlling the signal phase of the inverter is disclosed in U.S. Pat. No. 3,991,319, issued Nov. 9, 1976 to G. H. Servos et al. This patent discloses an uninterruptible power supply with standby power provided by an inverter circuit powered by a direct-current voltage source. DC current supplied by the reserve DC voltage source to the inverter is monitored and utilized to control the phase of the inverter signal relative to the commercial line signal. The DC current monitored is converted to a voltage which in turn is used to adjust the signal phase. This requires the generation of a voltage signal having a direct proportional relation to the current which requires integration over a period of time. In many applications a faster response than that permitted by integration is required.